pgsql: Use strlcpy instead of memcpy for copying the slot name in pgsta

Use strlcpy instead of memcpy for copying the slot name in pgstat.c.

There is no outright bug here but it is better to be consistent with the
usage at other places in the same file. In the passing, fix a wrong
assertion in pgstat_recv_replslot.

Author: Kyotaro Horiguchi
Reviewed-by: Sawada Masahiko and Amit Kapila
Discussion: https://postgr.es/m/20201104.175523.1704166915688949637.horikyota.ntt@gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/4f841ce3f7f4d429a3a275f82745d63c78cde4b2

Modified Files
--------------
src/backend/postmaster/pgstat.c | 12 +++++++-----
1 file changed, 7 insertions(+), 5 deletions(-)